A flush joint tubular connection has inner and outer diameters substantially the same as the tubing joints which the connection connects. A flush joint tubular connection made be the Hydril Company and covered by numerous patents comprise a first straight thread, a second straight thread of sufficient diameter to pass within the bore of the first thread and a tapered mating seat between the two joints of tubing which is a premium joint of high cost and according to published data, enjoys only 42% axial strength, relative to the pipe wall.
Standard A.P.I. non-upset tubing connections comprise couplings having outer diameters considerably larger than the pipe outer diameter but still only enjoy approximately 42% efficiency as above. A.P.I. does list a "turned down" collar outer diameter to increase clearance between strings, however, the "turned down" diameter still exceeds substantially, the pipe outer diameter.
No prior art discloses a flush joint tubular connection having tapered threads, that when properly assembled, effects optimum stresses within the small end of the external thread and within the large end of the internal thread so as to provide a connection of maximum efficiency. Conventional pipe connections have threads with like tapers and result in a constant diametrical interference along the taper between the external and internal threads, thereby causing excessive stresses or requiring increased wall thickness at the end of the pipe. Excessive stresses reduce the joint strength and an increased wall thickness rules out a flush joint connection.
It is therefore clear that a flush joint connection having a high efficiency as provided by the instant invention is needed for use within oilwells and other pipe assemblies wherein radial clearance is limited.
Standard pipe threads as well as A.P.I. threaded connections have such a tendency to cross-thread that "stabbing guides" are often used at a considerable cost of time and expense. Such threads have an extremely shallow stab depth and a relatively large thread depth, both of which add to the cross-thread problem. Perfect alignment is difficult to attain under normal field conditions and often impossible to attain under difficult conditions. Premium connections such as disclosed by Stone in U.S. Pat. No. 1,932,427 require even closer alignment to stab because of the close fit of straight threads and the "pin-nose" seal 32, which is highly susceptible to damage. To applicants belief no prior art comprised the combination of a deep stab, thread height and thread diameter as required to provide a tapered threaded connection that will stab easily and quickly without the possibility of cross-threading. By way of an example, a 23/8 EU 8rd A.P.I. tubing thread has a 2.473" pin end diameter and a 2.437" box bore at the first thread which allows no entry of the pin into the box at stab position. The counterbore of the box allows entry of the pin only 0.446" affording at best, axial alignment but no angular alignment so less than six degrees of angular misalignment will allow it to cross-thread.
About 1940, A.P.I. changed from 10V threads to 8rd and a substantial improvement resulted because less gauling occurred during makeup of the threads. It was then commonly assumed "that any thread finer than 8 threads per inch would gall and cross-thread" and that myth persists today. However, the improvement resulted almost entirely from the better thread form, eliminating the sharp edge V threads. The present invention with threads as fine as 20 per inch, run fast and smooth without cross-threading, and it has other features as well.
Conventional "near-flush" connections mentioned above, have two-step straight box threads formed within swaged-out ends and pin threads formed on swaged-in ends. Such swaged ends comprise a single tapered zone extending axially from the pipe body of original pipe diameters having a mean conical angle of taper of approximately two degrees. Typically, such swaged connections are rated by their suppliers as having from 50% to 75% efficiency depending on wall thickness, and with a variety of fluid pressure ratings. Such a swaged connection when compared to a 42% conventional flush joint connection, has improved strength, but at the expense of clearance.
To applicants best knowledge and belief, all such swaged connections now on the market are swaged to form only the degree of taper that approximates the lay of threads to be formed thereto. Typically, before a thread is machined in the tapered zone, a clean-up cut is made to assure there being enough metal to fully form the threads. Unfortunately, such a cut reduces the cross-section area of the tapered zone which limits connection efficiency. Additionally, production machining allows for only approximate axial positioning of the pipe in the machine prior to gripping the pipe in the chuck and such approximation can cause further thinning of the tapered zone. Thirdly, if first measurement of a freshly cut thread indicates that a thread recut is required, then the swage must be cut off and the end reswaged before even a 75% thread could be cut at that end. Therefore, in addition to the basic disadvantages of a two-step thread having a pin-nose seal, it is now even more clear why suppliers of pipe threads that are formed on swaged ends cannot provide a family of pipe connections with efficiencies greater than 75%.
Applicants U.S. Pat. No. 4,813,717 which is in the line of priority for the present application, discloses a connection with selective efficiency between 75% and 100% for non-upset pipe using a coupling in one embodiment per claims 1-17 and an integral connection in another embodiment per claims 18-19. The present invention is complimentary to said patent and teaches configurations for connections having swaged ends. To applicants best knowledge and belief, no non-upset integral connection is currently available that will meet the strain design criteria above. For users who prefer integral non-upset pipe connections, there is clearly a need for one with an efficiency sufficient to meet the strain design criteria defined above.
For purposes of this application, I define as follows: "Pin" is an externally threaded portion of a tubular member; "Box" is an internally threaded portion of a connecting member; "Flank angle" is the angle measured between a thread flank profile and the tubular axis; "Included angle" is the angle measured between the flanks, in the space between the flank surfaces; "Dope" is a pipe thread compound such as specified in API 5A2 that has been developed for 8Rd threads over many years to have an optimum combination of selected greases mixed with solid particles of specific dimension and nature so as to provide most desirable characteristics for sealing, lubricating and brushing over a substantial range of service temperatures and pressures. "Gap" is a distance that may exist between mating thread surfaces when they are positioned in best mating contact with each other, the distance being measured perpendicular to the surfaces.
The pipe thread form most widely used is the old "sharp-V" having 60 degree included angle as specified in ANSI B2.1 for AMERICAN NATIONAL STANDARD TAPER PIPE THREADS and in API 5B Table 2.8 for API LINE-PIPE THREADS. Although ANSI B2.1 shows thread sizes up to 24" and API 5B shows thread sizes up to 20", sizes above 41/2" are seldom used because:their high makeup torque makes field assembly impractical; such threads are very prone to handling damage; they frequently leak, loosen or break and are difficult to stab. Sharp-V threads are restricted to very low pressure services by government and industry codes such as API & ASME, who require the use of other connections such as flanged or welded, when dangerous fluids are to be contained. In addition to problems cited above, sharp-V threads often tear and gall during makeup which can cause excessive torque and worse, such as dangerous and costly leakage of fluid from within the pipe at some unpredictable time in the future. In an effort to solve such problems and because there is no reasonable alternative to the use of threaded pipe connections for downhole use in oil and gas wells, API adopted about 1940, the "8-Round" form shown in API 5B Table 2.9 which has 8 threads per inch and an included angle of 60 degrees, the flanks being connected by rounded roots and crests formed with a radii 0.017" and 0.020" respectively. Although thread tearing and galling were greatly reduced, the retained 60 degree included angle still allowed axial and bending loads to cause loosening, leakage and then pullout of a connection. For many 8Rd connections, pullout determines the parting load and leaks occur at much lesser loads. The 8 Rd thread form, without regard to pipe strength, is limited to only 5,000 psi service by API 5A due to such weaknesses. Both sharp-V and 8 Rd form standards specify intentional mismatches between crests and roots of mating threads like all other conventional threads known to applicant, which in turn, acts to increase the root gaps, which within tolerances, ranges from 0.005" to 0.011" for sharp-V and from 0.003" to 0.008" for 8Rd even after the mating flanks are wedged together at full makeup, which allows dope to leak through the root gap. As makeup begins, the root gap substantially equals the flank gaps, as dictated by the 60 degree included angle whereupon, solid particles in the dope extrude helically from between mating flanks and out of the connection as easily as it flows from the root gap. Thus, flanks wedge against each other with virtually no solid lubrication retained between them, which greatly increases galling tendency. The coefficient of friction with just grease is 0.084, vs 0.021 with the solid lubricants, which can increase torque by a factor of four. It is now clear how the root-gap/flank-gap ratio can affect torque.
To reduce thread pullout, API adopted many years ago, the "BUTTRESS THREAD FORM" depicted in API 5B FIGS. 2.5 & 2.6 for use on casing strings. The 87 degree tension flank angle greatly reduced tendency of pullout and an 80 degree compression flank angle reduced to a lesser extent, tendency for axial loads to jump the pin into or out of the box. However, such improvements were traded for a loss of sealing ability, a loss of rigidity and a cost increase as compared to API 8Rd threads. The Buttress form has many more dimensions to control than the 8Rd form which in turn, increases tolerance stack-up and results in flank gaps of 0.002" to 0.008". Even if a low pressure seal is formed on makeup, external loads imposed on such a connection cause end play between the mating flanks which extrudes Dope to cause loosening and leakage of the connection, particularly after the dope has had time to dry. Such end-play was felt necessary by the industry experts on API Committee 5B to prevent extreme torque and galling if "wedging" between the 13 degree included angle was allowed however, not obvious to them were the ill effects on connection rigidity and sealability that they incurred by the change. As a result, when critical jobs require both high strength and good sealability, the operator must use more expensive "pin nose" type connections that do not seal on the threads.
My force-vector analysis for flank-wedging mating threads having no root-crest contact, shows unit frictional resisting force: F=f(P)(1/sin T+1/sin C)/(1/tan T+1/tan C) where: f=coefficient of friction; P=interface pressure; T=tension flank angle; C=compression flank angle. When T=C then this equation reduces to F=f P/cos C, the conventional formula found on pages 3-28 and 3-29 of Marks Standard Handbook For Mechanical Engineers 8th ed which is correct when dope is allowed to extrude easily from between the load bearing surfaces. Mark shows "sin" instead of "cos" because that angle is referenced 90 degrees from mine. Since both Sharp-V and 8Rd threads have flank angles of 60 degrees, their torque is proportional to F=2 f P when calculated in accord with conventional practice.
API Committee 5B evidently thought that if they allowed flanks of the Buttress form to wedge like the 8Rd form, that torque would be proportional to F=8.2 f P, which is 4.4 times torque for the sharp-V or 8Rd form, and out of range for practical use. However, since API Buttress threads do not wedge, then F=f P=0.084 P. API Bulletin 5C3 entitled FORMULAS AND CALCULATIONS FOR CASING, TUBING, DRILLPIPE AND LINEPIPE PROPERTIES" gives many formulas, but they do not give a formula for torque because their test results were so erratic. The reason why API 8Rd connections have erratic torque is because a first connection may have large root gaps that will prematurely extrude dope, resulting in high torque and leakage while the next one may have small root gaps and much lower torque. However, a well may have hundreds of threaded connections and only one leaking connection can result in disaster. Pattersons connection may have a more consistant torque than 8Rd but because he does not wedge flanks, end-play will cause it to loosen and leak when it is subjected to repeated service loads. For many years, thread experts all over the world have used API 8Rd, Buttress and Pattersons threads, but none have recognized features and advantages of the present invention.
The API Buttress thread form allows Dope to extrude through gaps between the flanks which allow 0.002" to 0.008" end play at full makeup. Even if the gaps are reduced to between 0.002 and 0.004" per U.S. Pat. No. 4,508,375 to Patterson, end play will still occur when external loadings are imposed to cause loosening, extrusion and leakage at some unknown time later. Patterson also evidently knew that wedging of his threads would cause extreme torque, as evidenced by his 0.002" minimum gap allowed between flanks that prevents wedging. Had the API committee or Patterson recognized advantages of the present invention, they could have solved their loosening, leakage and torque problems.
API Specification 5CT on Tubular Specifications states in paragraph 5.19(a), "Pipe test pressures shall be held for not less than five seconds" which tests the pipe wall strength but does not test thread sealability because, it may take more than an hour for pipe dope to extrude through the thread gaps to prove a leak whereas, the required service life is generally between five and fifty years. Therefore, many casing connections are now on the market claiming high strength, good sealability and/or reasonable torque, but they use a separate pin-nose seal in addition to threads for holding the pin-nose seal together, which increases susceptibility to damage and increases cost, which in turn, prohibits their use for most applications. The resulting failures of pipe connections in deep wells increases rework costs, energy loss, danger to the public and damage to the environment.
The manufacture and use of large diameter pipe connections present several problems not encountered with small connections. Handling damage is much more probable and much more costly. Reimert U.S. Patent No. 4,429,904 discloses a large diameter welded on connection having special form Buttress threads that neither wedge nor seal. The O-ring 76 and stop shoulder 73 are mounted with a welded-on tubular member of increased diameter and radial thickness, at a great increase in cost. A reliable connection formed within dimensions of the pipe wall would save time, energy, cost and the uncertainty of weld quality. Reimert also reduces torque by preventing wedging of the thread flanks as shown in FIG. 9, but at a very high price. He also provides a seal separate from the threads by means of an O-ring and a pin-nose, but a seal that will degrade with time and that is not retrievable with the pin to the surface of the ocean.
Many patents such as U.S. Pat. No. 2,094,492 to Janata and U.S. Pat. No. 2,196,966 to Hammer disclose high angle tension flanks to wedge in cooperation with low angle compression flanks so as to keep torque within useable range. However, low compression flank angles sacrifice compressive strength of the connection which tends to cause loosening and leakage if subjected to compressive or bending loads. Others such as Patterson have high flank angles for both tension and compression flanks but do not allow wedging of the flanks that is necessary to prevent loosening and leakage.
Therefore, industry is clearly in need of a threaded pipe connection having low torque, that stabs easily, that will seal reliably and does not loosen when service loads are imposed on it.